The present invention relates to special high-cristobalite refractory cores for high-temperature directional solidification casting of superalloys and more particularly to the use of high-silica cores containing mineralizers to promote rapid devitrification of the vitreous silica to cristobalite.
Turbine airfoils and various other precision metal parts are formed of special metal alloys and are manufactured by a directional solidification casting process, commonly known as the "D.S. process," in which the parts are cast from conventional nickel- and cobalt-base alloys, such as B 1900, Mar-M-200, Mar-M-509, TRW 6A and the like. In order to produce hardware with complex internal cavities, preformed ceramic cores are employed to form the cavity in the casting. These cores are porous and preferably made predominantly of silica so that they can be removed readily by leaching without damage to the casting.
The cores are formed to a predetermined size and shape by molding a refractory core composition, and the green cores are then fired at a temperature of 1000.degree. to 1300.degree. C. to remove combustibles and to form a porous high strength product.
The first step in the production of a hollow turbine blade or other airfoil by a typical investment casting process is to place the preformed ceramic core in a cavity die and to inject wax or other destructible pattern material around the core. Sprue components including gates, downpoles and the like are also injected and, together with the cored patterns of the part desired, are assembled into clusters. The wax assembly is then dipped in a ceramic slurry, dusted with refractory grain, and dried. These steps are repeated many times until there is formed a shell of sufficient thickness, for example, a thickness of one-fourth inch. The process of making such a shell mold is well known and is fully described in U.S. Pat. No. 2,932,864.
After the desired number of layers have been formed on the shell mold and the mold has been thoroughly dried, the wax is removed by the application of heat. Autoclave and flash fire dewaxing are the most commonly used methods. After firing and cleaning, the mold is ready for use in metal casting.
In the standard casting process the molds and consequently the cores, are preheated to at least 800.degree. C. and the molten metal is poured into the molds at high temperatures. In the so-called "D.S. process", the molds are preheated to temperatures in the range of 1400.degree. to 1600.degree. C. and the metal is poured at high temperatures, such as 1500.degree. to 1650.degree. C.
In order for a refractory core to function acceptably in an investment casting process, an optimization of properties is required. The core composition must be suitable for economical molding and must be carefully selected so that the core is porous and can be leached out without damaging the metal casting, so that it has sufficient strength to resist the forces applied during wax injection, so that it has adequate high-temperature strength to withstand stresses due to non-uniform metal flow, so that it has dimensional stability during preheating and metal pouring, and so that it is chemically inert to the molten superalloys.
Core compositions were developed prior to the present invention which met the necessary requirements for standard investment casting. However, they were not entirely satisfactory for making cores used in the directional solidification process for casting turbine blades and vanes and other precision metal parts.
In such "D.S. process" the shell mold and its associated core can be preheated to a temperature of 1350.degree. to 1500.degree. C. or higher and a molten superalloy poured into the mold at a temperature of 1450.degree. to 1600.degree. C. The molten metal contacts a cooled chill plate, which supports the mold and the core, and the casting is progressively solidified and gradually cooled to a temperature below 1100.degree. C. by controlling the heat and by gradually lowering the chill plate away from the upper heating zone of the furnace. A typical cycle often requires one-half hour or more so that the core can be subjected to a temperature above 1450.degree. C. for a substantial period of time. This can cause serious sagging and distortion of a conventional porous silica core so that the necessary close tolerances cannot be maintained in the cast piece.
The refractory cores which were available prior to the present invention had poor thermal stability at high temperatures, such as 1550.degree. C. and above, and therefore were unsatisfactory or did not permit the D.S. process to be carried out at the optimum temperature or with the desired thermal gradients.
In a typical D.S. process the porous leachable refractory core and the surrounding shell mold are preheated to a temperature of 1350.degree. to 1500.degree. C. for ten minutes to one hour or so before the molten superalloy is poured or allowed to flow into the mold, and this necessarily results in some shrinkage while converting some of the fused silica to cristobalite.
Heretofore, the amount of impurities in silica cores was limited to avoid contamination of the metal during casting and to avoid loss of thermal stability. Small amounts of impurities such as sodium can, for example, cause formation of low melting glasses and drastically lower the resistance of a core to sagging under heat. It is found, for example, that a few percent of a contaminating material in a core containing 95 percent or more of essentially pure fused silica can lower the temperature at which plastic flow begins by more than 50.degree. Centrigrade.
Ordinarily, fused silica contains sodium and other normal impurities which promote devitrification, but the amounts are small and the impurities are distributed throughout the particles so that their effect is minimized. The devitrification of a conventional high-silica core proceeds at such a slow rate that the amount of cristobalite is limited and the core sags and deforms when it is used in the D.S. process for casting of superalloys. For this reason, silica cores made prior to this invention containing high percentages of silica did not have the thermal stability sought for use in the D.S. process and did not solve the sagging problem.
A satisfactory solution to the problem is not obtained by mixing silica particles with other refractories, such as alumina, because this does not produce a core with optimum refractoriness as desired in modern D.S. casting processes. The incorporation of such other refractories can limit the maximum use temperature so that the D.S. process cannot be carried out at desirable temperatures such as 1550.degree. to 1600.degree. C.